How do I group and count elements using groupBy and count in Kotlin?

By I Wayan Saryada in Kotlin 0 Comment

In Kotlin, you can group elements with groupBy, then count how many items are in each group.

Basic example

val words = listOf("apple", "banana", "apricot", "blueberry", "avocado")

val countsByFirstLetter = words
    .groupBy { it.first() }
    .mapValues { (_, words) -> words.count() }

println(countsByFirstLetter)

Output:

{a=3, b=2}

Here:

  • groupBy { it.first() } groups words by their first letter.
  • mapValues { it.value.count() } converts each grouped list into its size/count.

You can also write it as:

val countsByFirstLetter = words
    .groupBy { it.first() }
    .mapValues { it.value.size }

Counting objects by a property

data class Person(val name: String, val city: String)

val people = listOf(
    Person("Alice", "London"),
    Person("Bob", "Paris"),
    Person("Charlie", "London"),
    Person("Diana", "Paris"),
    Person("Eve", "Berlin")
)

val peopleByCity = people
    .groupBy { it.city }
    .mapValues { it.value.count() }

println(peopleByCity)

Output:

{London=2, Paris=2, Berlin=1}

More efficient option: groupingBy + eachCount

If you only need counts, prefer:

val countsByFirstLetter = words
    .groupingBy { it.first() }
    .eachCount()

This avoids creating intermediate lists for every group.

println(countsByFirstLetter)
// {a=3, b=2}

So the common choices are:

items.groupBy { key }.mapValues { it.value.count() }

or, more efficiently:

items.groupingBy { key }.eachCount()

How do I count the number of occurrences of a char in a String?

By I Wayan Saryada in Core API, Lang Package 0 Comment

This example show you how to count the number of a character occurrences in a string. We show two ways to do it, using the String.replaceAll(String regex, String replace) method and creating a loop that check every char in the String and count the matched char.

package org.kodejava.lang;

public class CharCounter {
    public static void main(String[] args) {
        String text = "a,b,c,c,e,f,g,g,g,g,h";

        // Use the CharCounter.countCharOccurrences() method to count.
        int numberOfLetterC = CharCounter.countCharOccurrences(text, 'c');
        System.out.println("Letter c = " + numberOfLetterC);

        // Other solution is to use the String.replaceAll() method. We'll
        // replace the chars other than the counted char with an empty string.
        // To get the char occurrences we count the length of the remaining
        // string.
        int numberOfComma = text.replaceAll("[^,]", "").length();
        System.out.println("Comma    = " + numberOfComma);

        int numberOfLetterG = text.replaceAll("[^g]", "").length();
        System.out.println("Letter g = " + numberOfLetterG);
    }

    /**
     * Count number of specified char occurrences in the specified string.
     */
    private static int countCharOccurrences(String source, char target) {
        int counter = 0;

        // Loop through the string and increment the counter if the
        // target character found in the string. 
        for (int i = 0; i < source.length(); i++) {
            if (source.charAt(i) == target) {
                counter++;
            }
        }
        return counter;
    }
}